The availability of micromachining technology has permitted the development of new classes of semiconductor-based microdevices for a variety of applications. Such technology, when based on batch processing with standard semiconductor clean room processes, often provides excellent economies in per unit cost and enhanced reliability for the end product. Micromachining provides the advantages of: design flexibility, ability to make structures of micron and submicron dimensions, batch fabrication at low cost, reliability, reproducibility, easy fabrication of arrays, ability to include control and signal conditioning electronics on a same substrate, and the use of a well established manufacturing infrastructure.
Micromachining with standard semiconductor processing techniques has been used to produce thermally isolated microplatforms which find a number of uses including: calorimetric gas detection, flow sensors, uncooled infra-red sensors, etc. The state of the art for thermally-isolated platforms is described by Manginell, Smith and Ricco in "An Overview of Micromachined Platforms for Thermal Sensing and Gas Detection", Proc. of the SPIE Conference on Smart Electronics and MEMS, Volume 3046, pages 273-383, March, 1997. Manginell et al. describe microplatforms that are suspended above substrate cavities and/or released by the sacrificial etching away of underlying film structures.
Fung et al. in "Thermal Analysis and Design of a Micro-Hotplate for Integrated Gas Sensor Applications" Proc. of the 8th Int;l Conf. on Solid-State Sensors and Actuators, Stockholm, Sweden, pages 818-821, 1995, describe a microhotplate for an integrated gas sensor application. Therein is described a heated microplatform that is thermally isolated from an underlying substrate by suspension microbeams and a micromachined underlying cavity space.
The various applications for thermally-isolated microplatforms are enhanced by controlling the rate of heating or cooling of a microplatform or microplatform array. A suspended microplatform derives thermal isolation from three phenomena which can either cool or heat the microplatform depending upon the specific structures. These phenomena are: radiation, convection, and conduction.
The total power P radiated per unit area of a microplatform is given by EQU P.sub.bb =K.sub.1 A.sub.bb T.sup.4
where k.sub.1 is a constant depending upon the specific surface of the microplatform, A is the effective platform area, and T is the temperature in absolute degrees. Thermal conduction can occur through a gaseous ambient and through thin film support microbeams. The total power conducted through the gaseous ambient or the microsupport beams is given by EQU P.sub.co =k.sub.2 A.sub.co (T.sub.2 -T.sub.1)
where k.sub.2 is a constant depending on the specific gas or solid-state film, A.sub.co is the effective area cross section for heat flow, and (T.sub.1 -T.sub.2) is the differential temperature across which the heat conduction occurs.
The total power flowing away from the microplatform by convection is given by the expression EQU P.sub.cv =k.sub.3 A.sub.cv (T.sub.2 -T.sub.1)
where k.sub.3 is a constant depending upon the gas ambient and the surface of the microplatform, A.sub.cv is the effective area of the microplatform exposed to convection heat flow, and (T.sub.2 -T.sub.1) is the temperature differential between the surface of the microplatform and the ambient gas.
When a microplatform is positioned very close to an underlying substrate, there is a heat flow from the warmer structure to the cooler structure that is typically dominated by a combination of thermal conductivity and gas convective thermal transport. This heat flow decreases as the physical separation between the substrate and the microplatform increases. For microplatforms operating in a gas ambient (atmospheric gas pressure, for instance) substantial increases in thermal isolation of the microplatform can be obtained by increasing the distance between the microplatform and the underlying substrate.
When a support microbeam structure is specifically designed to greatly reduce the thermal conductivity through the microbeams, then increased height of the microplatform over a substrate has an even greater effect of increasing the thermal isolation of the microplatform.
For microplatforms designed for applications such as structural supports for bolometers and thermopiles, a maximum thermal isolation of the microplatform is usually desired so as to increase the sensitivity of the microplatform to incident radiation, generally in the infrared. A temperature sensor mounted on such a microplatform operates with maximum temperature sensitivity to incident radiation when the microsupport beams possess a minimum thermal conductivity. Also for such designs, the thermal isolation of the microplatform is increased further by increasing the separation between the microplatform and the underlying substrate.